In recent years, in a field of cylinder-type dampers used in a vehicle suspension system, various types of variable damping force dampers have been developed to variably control the damping force depending on dynamic states of the vehicle in order to achieve both of favorable riding comfort and high driving stability. In the past, variable damping force dampers of a mechanical type were common in which a rotary valve is provided to the piston to variably control a flow area of an orifice and the rotary valve is rotatably driven by an actuator. However, in order to simplify the structure and improve the control response time, variable damping force dampers of an MRF type have been developed in recent years, in which a magneto-rheological fluid (MRF) is used as an operating oil and the piston is provided with a magnetizable liquid valve (MLV) for controlling an apparent viscosity of the MRF (see United States Patent Application Publication No. 2006/0224285, for example).
In general, in a damper control apparatus for controlling the MRF-type variable damping force damper, a target damping force setting means sets a target damping force for each wheel according to a lateral acceleration, longitudinal acceleration, etc. of the vehicle body, and thereafter, a target current setting means sets a target current for each MLV according to the target damping force and a stroke speed of the damper. Typically, the lateral acceleration, longitudinal acceleration, damper stroke speed, etc. are detected by various sensors (lateral G sensor, longitudinal G sensor, stroke sensor, etc.) disposed at appropriate portions of the vehicle body and suspension system, and the detection signals from the sensors tend to include various high frequency noises. For this reason, it has been proposed to use a low-pass filter having a predetermined cut-off frequency to remove or reduce the high frequency noises from the detection signals of the sensors or target current, thereby improving the control reliability (see Japanese Patent Application Publication No. 7-25257 and Japanese Patent Application Publication No. 6-278638, for example).
The low-pass filters disclosed in JPA No. 7-25257 and JPA No. 6-278638 serve to remove high frequency components from the detection signals with a prescribed response characteristic, but there was a problem arising therefrom. For example, when an automobile is running meanderingly on a road having surface irregularities, the detection signal from the stroke sensor (e.g., stroke speed) may oscillate at a frequency near a sprung mass resonance frequency and the direction of the stroke speed, which can be expressed by positive and negative signs, may change accordingly, as show in FIG. 11(a). If the low-pass filter is given a low response characteristic, the damping force (or the target current) tends to overshoot at the points where the direction of damping force changes as shown in FIG. 11(b), and this can deteriorate the riding comfort. On the other hand, when an automobile is turning steadily on a flat road surface, for example, the detection signal from the stroke sensor is generally kept near zero as shown in FIG. 12(a), but high frequency noises generated from the lateral G sensor or stroke sensor itself are sometimes added onto the detection signal. In such a case, if the low-pass filter is given a high response characteristic, an amount of reduction of the damping force caused by the high frequency noises can be undesirably large, and this may unfavorably affect smooth attitude control of the automobile and deteriorate the driving stability.